1. Field of the Invention
The present invention relates generally to laser systems and more specifically to tunable solid-state lasers and techniques for practical realizations of high efficiency, power scalable lasers.
2. Description of Related Art
Tunable lasers in the near infrared region between 700 and 950 nm are highly desirable for medical applications and for short pulse generation. In addition, frequency conversion to higher harmonics of the fundamental harmonic allows accessing the a wide spectral region ranging from the deep ultraviolet to the blue. Among the different tunable lasers available at this wavelength range, Cr doped (Cr3+) materials such as chrysoberyl (Cr3+:BeAl2O4 or Alexandrite), fluorides such as LiSAF and LiCAF and Ti-doped sapphire are commercially available. Ti:sapphire has the broadest spectral output and high gain but suffers from short lifetime (a few microseconds) which makes it unsuitable for diode pumping. The Cr-doped fluorides have lifetime near 100 ms and have been successfully pumped by diodes but still require improvements in material quality and growth techniques. Alexandrite, on the other hand, is a well-known developed laser material with peak output near 752 nm and demonstrated broad tunability from about 725 to just over 800 nm as was already shown by Walling et al (see IEEE J> Quant. Electron. QE-16, 1302, 1980). With frequency doubling and tripling of laser radiation it can produce useful tunable output from about 240 to 400 nm, a region of great current interest for potentially high volume applications including micromachining, biophotonics and biological detection. Alexandrite lasers have demonstrated power scalibility in both CW and Q-switched operational mode, and are particularly useful in applications requiring higher pulse energies than is available, for example from Ti-doped sapphire lasers. In particular, Alexandrite, in a Q-switched mode, offers the possibility of replacing bulky and inefficient excimer lasers, while offering tenability in the UV-to-blue spectral regimes.
The material is thermally very robust and has been available in a flashlamp-pumped package for many years now. However, flashlamp pumped Alexandrite lasers have very low efficiency—typically less than 1%—and a diode-pumping approach was considered to be a highly desirable approach to achieve higher energy. Since Alexandrite is an orthorhombic crystal that is optically biaxial, it has an absorption spectrum that is different for light polarized parallel to the a, b, and c crystalline axes, with the absorption strongest for pump light electric field parallel to the b axis, peaking at around 600 nm. Because of this strong absorption in the red spectral region (600-650 nm), AlGaInP diode lasers with emission around 640-645 nm have been successfully used to diode pump Alexandrite, as was shown by Scheps et al in Opt. Comm. 97, 363, 1993. These experiments produced however relatively low slope efficiencies (24-26%) despite high absorption of the diode light, a result attributed to Alexandrite's low gain resulting from a relatively small stimulated-emission coefficient. For example at 753 nm and room temperature it is valued at σe=0.5×10−20 cm2, which is low compared to, e.g., the 1064 nm transition in Nd:YAG where σe=3.1×10−19 cm2 or Nd:YVO4 which is 16×10−19 cm2. This means that to reach threshold, Alexandrite must be pumped at a very high intensity. Indeed, a much higher 64% slope efficiency was achievable in the experiments of Scheps et al by pumping the same alexandrite rod with a high-brightness, near diffraction limited dye laser operating at 645 nm along with a threshold lower by about a factor of 2. As derived from these experiments, with a dye laser pump intensity incident on the Alexandrite crystal of about ˜5.6 MW/cm2. the average round-trip net gain produced was only about 0.0190 at threshold increasing to 0.083 at the full pump power of 300 mW. This contrasts with a typical incident intensity on a Nd:YVO4 laser end-pumped with a 1 W diode at 808 nm of only about 6.4 kW/cm2, i.e., a factor of 100-1000 times less than that required for Alexandrite. The modest gain of Alexandrite means that high pump intensities are essential as well as low-loss resonators with high outcoupler reflectivities.
It is, however, very difficult to obtain the requisite high intensities from diode lasers suitable for pumping Alexandrite. The main difficulty involves the small pump spot diameters required for pumping Alexandrite. Except for low power single mode diodes, higher power bars or diode arrays do not possess the requisite beam quality to enable sufficiently high incident pump intensities. Most diode lasers have very good beam-quality only in the direction perpendicular to the diode stripe, while in the direction parallel to the stripe, the divergence is much higher, resulting in an overall poor beam-quality which prevents focusing to a sufficiently small spot diameter in at least the one dimension. In the experiments of Scheps et al for example, for example, diodes with a stripe width of 60 μm were focused by a 5 cm lens to a spot size of about 10 μm by 18 μm. In contrast the dye laser used in the same report, with its circularly symmetric beam, had a spot diameter of less than 10 μm, resulting in higher pump intensity.
Furthermore, focussing the pump to a small waist at one end of the gain rod means that the spot size increases progressively as it propagates through the rod, making it difficult to maintain the desired pump intensity throughout the length of the gain medium.
Scaling up to the watt or tens of watts level using an approach based on available red single-mode diode powers does not appear to be promising, at least based on current technology. The existing art in this field is therefore deficient in providing practical solutions to the problem of constructing practical efficient and scalable tunable laser devices based on relatively low gain materials such as Alexandrite. There is in particular a need to provide constructions suitable for end pumping of a tunable laser medium that are compatible with power scaling and can be applied to many different media and geometries without introducing undue complexities.